Improving the fuel efficiency of automobiles has recently become an important issue from the viewpoint of global environment conservation. In order to achieve compatibility between fuel efficiency and crash safety of automobiles, active efforts have been made to reduce the weight of car bodies themselves by reducing the thickness of parts while increasing the strength of materials used. While hot rolled steel sheets heretofore used for automobile parts have a grade in terms of tensile strength of 440 MPa or 590 MPa, there has recently been an increasing need for high strength hot rolled steel sheets having a grade of 780 MPa or higher.
However, increasing the strength of steel sheets is generally accompanied by a decrease in formability. Thus, various attempts have been made in order to improve formability such as bendability required for steel sheets to be used as automobile parts and the like.
For example, patent document 1 describes a hot rolled steel sheet which has a steel composition containing, in terms of mass, C at more than 0.055% and less than 0.15%, Si at less than 1.20, Mn at more than 0.5% and less than 2.5%, Al at less than 0.5%, P at less than 0.1%, S at less than 0.01%, N at less than 0.008%, and one or two or more selected from the group consisting of V at more than 0.03% and less than 0.5%, Ti at more than 0.003% and less than 0.2%, Nb at more than 0.003% and less than 0.1% and Mo at more than 0.03% and less than 0.2% while satisfying Expression (1) below, the balance being represented by Fe and inevitable impurities, and which includes a steel microstructure that contains not less than 70% by volume of equiaxed ferrite with a Vickers hardness Hvα defined by Expression (2) below and 0 to 5% by volume of martensite, the balance being represented by one, or two or more selected from ferrite excluding the equiaxed ferrite, bainite, cementite and pearlite.0.04<C—(Ti−3.43N)×0.25−Nb×0.129−V×0.235−Mo×0.125<0.05  (1)Hvα≧0.3×TS+10  (2)
According to the technique described in patent document 1, the hot rolled steel sheet configured to contain hard equiaxed ferrite at not less than 70% exhibits excellent bendability with a critical bending radius of not more than 0.5 mm in the case of a tensile strength of not less than 850 MPa and a sheet thickness of 1 mm.
Patent document 2 describes a method for manufacturing high strength hot rolled steel sheets having a tensile strength of not less than 780 MPa, which includes hot rolling a steel slab with a steel composition containing, in terms of mass %, C at 0.04 to 0.15%, Si at 0.05 to 1.5%, Mn at 0.5 to 2.0%, P at not more than 0.06%, S at not more than 0.005%, Al at not more than 0.10% and Ti at 0.05 to 0.20% at a finish temperature of 800 to 1000° C., thereafter cooling the steel sheet at a cooling rate of not less than 55° C./s and subsequently at a cooling rate of not less than 120° C./s for the temperature range of 500° C. and below so as to cool the steel sheet by nucleate boiling cooling, and coiling the steel sheet at 350 to 500° C.
According to the technique described in patent document 2, a high strength hot rolled steel sheet with a tensile strength of not less than 780 MPa is obtained which has a microstructure including more than 95% of bainite and less than 5% of irreversibly-formed other phases and exhibits excellent stretch flangeability after working as well as stability with small variations in quality within the steel sheet.